To begin with, it really does handle standard-conforming web services. Several of the other packages out there lack true support for XML namespaces and XML schema. Some of them require, for example, that the messages use fixed prefixes for XML namespaces, contrary to the XML spec. It handles some of the more complex XML schema cases (xs:any or xs:anyType), though there are some extreme cases (xs:choice) that it doesn't do.
Along with the rest of CXF, it comes under an Apache license, so that those of you with an allergy to the GPL need have no fear.
It handles MTOM attachments. Why would you want MTOM attachments in a browser, where you can't have any binary at all? You would want them to attach non-XML-1.0 text without paying the base64 tax.